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A046451 Semiprimes whose prime factors, when concatenated, yield a palindrome. 1
4, 9, 25, 39, 49, 69, 119, 121, 129, 159, 219, 249, 259, 329, 339, 403, 469, 669, 679, 1207, 1329, 1589, 1703, 2319, 2321, 2359, 2649, 2701, 3039, 3421, 3503, 3629, 3639, 3899, 4303, 4607, 4839, 5603, 5739, 6209, 6401, 6979, 7269, 7663, 7819, 8203, 8519 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

LINKS

Harvey P. Dale, Table of n, a(n) for n = 1..1000

EXAMPLE

3899 = 7 * 557 -> 7557 is palindromic.

MATHEMATICA

d[n_] := IntegerDigits[n]; co[n_, k_] := Nest[Flatten[d[{#, n}]] &, n, k - 1]; Select[Range[8520], PrimeOmega[#] == 2 && Reverse[x = Flatten[d[co @@@ FactorInteger[#]]]] == x &] (* Jayanta Basu, Jun 26 2013 *)

Select[Range[9000], PrimeOmega[#]==2&&PalindromeQ[FromDigits[ Flatten[ IntegerDigits/@FactorInteger[#][[All, 1]]]]]&] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Mar 19 2020 *)

CROSSREFS

Cf. A046447.

Sequence in context: A063760 A238334 A130448 * A158142 A158143 A082200

Adjacent sequences:  A046448 A046449 A046450 * A046452 A046453 A046454

KEYWORD

nonn,base

AUTHOR

Patrick De Geest, Jul 15 1998

EXTENSIONS

Edited by Charles R Greathouse IV, Apr 28 2010

STATUS

approved

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Last modified August 14 10:47 EDT 2020. Contains 336480 sequences. (Running on oeis4.)